The Book of Inkscape

10.8 Patterns

A pattern is a paint type (8.1.1 Paint) where an object’s fill or stroke consists of repeated copies of a tile. The tile can be anything: a single object (e.g., a bitmap) or a group of objects, using any style properties or Inkscape techniques. In this regard, patterns are a very rich and flexible feature. In another respect, however, patterns in SVG are quite limited: They can only use a simple rectangular grid of tiles with no rotations or reflections (compare, e.g., with the clone tiler which implements 17 different symmetry types, 16.6 Tiling Clones).

You cannot have both a gradient and a pattern on an object; they are two alternative paint types. If you want a transparency gradient on an object with pattern, read 10.7 Gradient ...